Not always... Cub Swanson and Brian Ortega are both black belts. I mean you can't cap someones skill level only based on what belt they have, that's pretty much a fact.
Also how someones BJJ translates to MMA can be a big difference even if they would be competetive in a pure BJJ fight. The same goes for wrestling and other martial arts. On paper Patrick Cummins for example should be able to dominate a lot of fighters with his wrestling, but that's not the case, he hasn't been able to transfer his skills over and make it work when it's not just a wrestling match
edit. and I was a bit sassy because of the voodoo comment